Driven by ever-increasing requirements for improved fuel efficiency, reduced undesirable emissions and reduced noise, future aero-engines and land-based turbines will have to incorporate new systems to monitor the turbine conditions, analyse the incoming data and modify operating parameters to optimise operations and thus achieve improved performance. Sensing technology is the foundation upon which such systems are based. New sensors that are able to operate under the harsh environment present in a turbine must be developed to enable the measurement of previously unmeasurable parameters critical for monitoring the overall health of the turbine. The potential benefits from those systems are significant and may be categorised into two primary areas: turbine efficiency and turbine maintenance. For both purposes, the monitoring of the operational state of the turbine's rotor blades is an essential requirement, in particular for a detailed understanding of the functioning and health of a turbine.
In order to perform an accurate measurement of the operational state of the rotor blades, a microwave sensor is typically mounted through a hole or attached to the inside of the engine case to enable the microwave sensor to cast its beam onto the blades, which will be rotating and subsequently passing by the sensor during engine operation. Yet the environment encountered in turbine engines is harsh with gas path temperatures exceeding 1300 K in high-pressure turbines and temperatures around 900 K in the rear stages of a high-pressure compressor of aero-engines, most often with a high thermal gradient as well. It is a dirty environment with oil, combustion by-products and other contaminants. A sensor being able to operate reliably at those extreme temperatures and in such a harsh environment is therefore a key component.
A sensor to solve this problem has been addressed in patent application No. US 2010/0066387 A1 which discloses a device for determining the distance between a rotor blade and a wall of a gas turbine surrounding the rotor blade. The device comprises a waveguide that guides electromagnetic waves with at least two frequencies. Waves with one of the frequencies are emitted from the sensor and reflected back by the rotor blade and waves with the other frequency are reflected by a sealing element at the end of the waveguide. The distance of the device with respect to the rotor blade is then determined by comparing phases of the waves.
A disadvantage of this sensor is that a temperature dependent expansion of the turbine walls, in which the device is mounted, also causes a corresponding expansion of the waveguide, making the desired distance determination inaccurate. The problem may be circumvented by subtracting the phase comparison values of the waves of the two frequencies from each other and assigning this value to a previously measured value for the distance under those temperature conditions, e.g. on the basis of a value table. Nevertheless, a more direct way of accounting for the temperature changes would be highly desirable in order to make sure that the momentary operating conditions of the turbine are met, which generally do not solely dependent on the temperature gradient. Apart from the reliability of the measurement, it would also be desirable to improve the detection performance, in particular to increase the measurement resolution, and to extend a monitoring of the rotor blades beyond the detection of the single parameter of a distance measurement in between the blade tips and the sensor.